This invention relates to a front bicycle derailleur fork for controlling the selective engagement of a transmission chain of a bicycle on a plurality of chainrings of different diameters associated to the bottom bracket of the bicycle, said fork comprising an inner plate and an outer plate, which is essentially parallel and distanced with respect to the inner plate, and joined to it by an upper bridge portion, said outer plate being located on the side of the larger diameter chainring and said inner plate being located on the side of the smallest diameter chaining.
FIG. 1 of the annexed drawings shows a perspective view of a front bicycle derailleur of the traditional type, comprising a fixed body 1 with a fork 2 for anchoring to a tube 3 of the bicycle frame near the bottom bracket. The derailleur 1 also comprises a fork 4 for controlling the selective engagement of the bicycle chain (not illustrated) on the chainring (not illustrated) associated to the bottom bracket. The fork 4 comprises an inner plate 5, closer to the bicycle frame, and an outer plate 6, facing outwards, which is parallel and distanced with respect to the plate 5. The two plates 5, 6 are connected by an upper bridge 7 and a rear bridge 8. The upper bridge 7 is provided with a pair of parallel tabs 9 for the articulated connection to a rocker arm 10, which is articulated to the fixed body 1 of the derailleur in point 11 and presents an opposite end 12 for anchoring the end of the derailleur actuating flexible wire (not illustrated). The inner plate 5 also presents a pair of parallel tabs 13 for the articulated connection to a second articulated arm 14, whose opposite end is connected in an articulated fashion to the fixed body 1 in point 15. The two arms 10, 14, consequently, define an articulated parallelogram connection, which allows the fork 4 to move with respect to the fixed body 1 in a direction which is essentially transversal with respect to the longitudinal direction of the bicycle, indicated by the arrow A. A spring 16, which tends to recall the fork 4 forwards an end of travel position (leftwards with reference to the drawing), is provided on the spindle of the inner plate 4 and the arm 14. From this position, the fork can be moved rightwards (always with reference to FIG. 1) following the intervention of a derailleur control (not illustrated) whereby pulling the arm 10 by means of the flexible wire connected to the end 12.
In this description and in the claims which follow, the expressions “inner plate” and “outer plate” refer to the usual assembly condition of the front bicycle derailleur. According to the traditional assembly arrangement, the set of chainrings associated to the bottom bracket of the bicycle is arranged to the right of the bicycle frame with the largest diameter outside and the smallest on the inside. Similarly, the fork 4 presents an inner plate 5 facing the bicycle frame and an outer frame 6 facing outwards with respect to the bicycle.
It is essential to note that during the front derailleur operation, one of the two plates 5, 6 of the derailleur pushes the chain laterally to shift it from the chainring it is meshed with to the next chainring. More precisely, the lateral pushing action of the chain is exerted by the inner plate 5, when the chain is shifted from a smaller diameter chainring to a larger diameter chainring, and by the outer plate 6, when the chain is shifted from a larger diameter chainring to a smaller diameter chainring.